The U-Boat Enigma

In response to:

In his insightful review of the German official history of World War II [“Germans Confront the Nazi Past,”NYR, February 26], Max Hastings says that the number of Allied merchant vessel sinkings “fluctuated in remarkably exact inverse correlation” with British codebreaking successes. Not so.

In May 1941, solution times for an Enigma-enciphered message averaged three or four days. In June and July, captured documents reduced that to three or four hours. But when those keys expired in August, solution times rose again. Yet the U-boats sank roughly the same tonnage in both the fast-solution and the slow-solution months. More escorts were available and they accompanied convoys all across the Atlantic. Their greater experience made them more efficient. Air cover increased. The speed of ships sailing independently rose from thirteen to fifteen knots. Some U-boats had been withdrawn to fight Soviet shipping, leaving fewer in the Atlantic. The new U-boat crews were less experienced. And Hitler wanted to avoid clashes with American warships. All these elements destroyed any tight correlation between codebreaking and shipping success.